1. Field of the Invention
This invention relates to propulsion systems. More specifically, the invention is an atomic-based propulsion system and method for operating same.
2. Description of the Related Art
Establishing routine, low-cost access to Earth""s orbit is critical to mankind""s future in space. The excessive costs of launching even small payloads has severely limited activities in Earth""s orbit and restricted the ambitiousness of interplanetary missions. Truly low-cost access to space will require launch systems having low propellant fractions (i.e., the propellant fraction xcex=propellant mass/gross vehicle mass) and the operational flexibility of modern passenger aircraft.
For years, it was believed that progress made in materials technology since the early space program (e.g., composites, etc.) would enable very lightweight vehicle structures and use of chemical propulsion for single-stage-to-orbit (SSTO) operation. Other enhancements, such as aerospike nozzles and tri-propellant engines, improved chemical rocket performance over the flight envelope. However, even with the improvements in material and chemical-rocket technologies, the propellant fractions of today""s space vehicles are simply too high (xcex=0.9) and result in vehicles with a ratio of payload mass to gross vehicle mass of less than 0.1.
More recently, chemically-powered rocket based combined cycle (RBCC) propulsion systems have been considered. These systems integrate several rocket and air breathing propulsion modes into one engine. Because the specific impulse (or ISP as it is known) of the air breathing modes is much higher than a basic rocket, the effective mission specific impulse (i.e., average performance) is greater than other SSTO concepts. One of the simplest RBCC concepts is illustrated schematically in FIG. 1. A chemically-powered rocket 10 is typically one that burns a mixture of liquid hydrogen (H) and liquid oxygen (O). Rocket exhaust 12 is predominantly a hot water vapor and is expelled into a nozzle 14. Air 16 is inducted into nozzle 14 where it mixes with exhaust 12 as indicated at 18. In order to gain additional thrust, mixture 18 must combust. Such combustion can be achieved in one or a combination of two ways. If rocket 10 is operated efficiently so that exhaust 12 is mostly hot water vapor, additional hydrogen can be added to mixture 18 as supplied from a hydrogen fuel tank 20. Alternatively, or additionally, rocket 10 can be run in a fuel-rich mode so that exhaust 12 has excess hydrogen contained therein. However, since this condition also lowers the temperature of exhaust 12, one or more ignition devices 22 (or flame stabilizers as they are also known) are used to raise the temperature of mixture 18 to the point of combustion. Obviously, both of these options require apparatus and/or fuel which adds to the propulsion system""s weight, cost and complexity.
While systems such as that illustrated in FIG. 1 have improved thrust efficiency (i.e., increased ISP specifications), propellant mass in chemically-powered rockets makes their propellant fractions exceed that which would be desirable for an airliner-type of vehicle. Further, to take advantage of the increased specific impulse, chemically-powered rockets must operate in the air-breathing mode well up into supersonic speeds of Mach 8 or greater. However, by requiring the vehicle to operate at higher speeds within the atmosphere to support the air-breathing mode of the vehicle, aerodynamic heating of the vehicle""s surface becomes a major concern. For example, vehicle drag increases due to friction between the external air flow and the vehicle""s surface. This not only detracts from thrust performance, but also greatly increases the amount of heat deposited on the vehicle""s surface. In addition, aerodynamic heating can cause vehicle surface temperatures to exceed the surface material""s temperature limits thereby requiring the use of a heavy active cooling system to dispose of the heat.
Accordingly, it is an object of the present invention to provide a propulsion system and method that offers both improved thrust efficiency and low propellant fractions.
Another object of the present invention is to provide a propulsion system and method that can make use of lightweight fuel.
Still another object of the present invention is to provide a propulsion system and method that uses a single rocket stage.
Yet another object of the present invention is to provide a propulsion system and method that changes modes at increasing speeds and altitudes to maximize efficiency.
A still further object of the present invention is to provide a propulsion system and method that reduces aerodynamic heating concerns by lowering the velocity at which the system operates in an air-breathing mode.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method and system are provided for propelling an aerodynamic vehicle into space. An aerodynamic vehicle has a nuclear-based thermal rocket (NTR) propulsion system capable of producing a hydrogen exhaust. When the NTR propulsion system is operated, a thrust force is applied to the vehicle so that the vehicle is launched from a static condition and is propelled through the air. A flow of air is introduced into the hydrogen exhaust to augment the thrust force at speeds of the vehicle up to approximately Mach 6. At slower speeds, thrust augmentation is primarily the result of air/exhaust mixing, although some chemical combustion may also be taking place. At higher speeds, thrust augmentation is primarily the result of combustion of the air/exhaust mixture. The flow of air introduced into the hydrogen exhaust is adjusted based on the speed and altitude of the vehicle. When the speed of the vehicle is approximately Mach 6 and the altitude of the vehicle is approximately 40 kilometers, the flow of air is stopped.